There is not a single answer for "what is the next number" in this sequence. A sequence can have multiple rules. As long as the rule makes sense (works) it is valid.

The next number could be 54. The rule that would make the number 54 is "add three to the first number and 4 to every number after the first number".

The next number could be 55. The rule that would make the number 55 is "add the number that equals the tens digit to each number." 39 has 3 as the tens digit, so the next number is 39+3 = 42. All the numbers in the 40's have 4 as the tens digit, so add 4 to each one - 42+4 = 46, 46+4 = 50.

50 has 5 in the tens digit. 50+5 = 55.

There may be other valid rules that result in a different next number.